Build: Bump GNU Gettext version requirement to 0.19.

It silences a few warnings and most people probably have
0.19 even on stable distributions.

Thanks to Christian Kujau.
diff --git a/configure.ac b/configure.ac
index 7a22bf2..3df4348 100644
--- a/configure.ac
+++ b/configure.ac
@@ -609,7 +609,7 @@
 
 echo
 echo "Initializing gettext:"
-AM_GNU_GETTEXT_VERSION([0.18])
+AM_GNU_GETTEXT_VERSION([0.19])
 AM_GNU_GETTEXT([external])